The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with target diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases mediate intracellular signal transduction. They do this by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis and regulation of cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase comprised of α and β isoforms that are each encoded by distinct genes [Coghlan et al., Chemistry & Biology, 7, 793-803 (2000); Kim and Kimmel, Curr. Opinion Genetics Dev., 10, 508-514 (2000)]. GSK-3 has been implicated in various diseases including diabetes, Alzheimer's disease, CNS disorders such as manic depressive disorder and neurodegenerative diseases, and cardiomyocyte hypertrophy [see, e.g., WO 99/65897; WO 00/38675; Kaytor and Orr, Curr. Opin. Neurobiol., 12, 275-8 (2000); Haq et al., J. Cell Biol., 151, 117-30 (2000); Eldar-Finkelman, Trends Mol. Med., 8, 126-32 (2002)]. These diseases are associated with the abnormal operation of certain cell signaling pathways in which GSK-3 plays a role.
GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. These include glycogen synthase, which is the rate-limiting enzyme required for glycogen synthesis, the microtubule-associated protein Tau, the gene transcription factor β-catenin, the translation initiation factor e1F-2B, as well as ATP citrate lyase, axin, heat shock factor-1, c-Jun, c-myc, c-myb, CREB, and CEPBα. These diverse targets implicate GSK-3 in many aspects of cellular metabolism, proliferation, differentiation and development.
In a GSK-3 mediated pathway that is relevant for the treatment of type II diabetes, insulin-induced signaling leads to cellular glucose uptake and glycogen synthesis. GSK-3 is a negative regulator of the insulin-induced signal in this pathway. Normally, the presence of insulin causes inhibition of GSK-3-mediated phosphorylation and deactivation of glycogen synthase. The inhibition of GSK-3 leads to increased glycogen synthesis and glucose uptake [Klein et al., PNAS, 93, 8455-9 (1996); Cross et al., Biochem. J., 303, 21-26 (1994); Cohen, Biochem. Soc. Trans., 21, 555-567 (1993); and Massillon et al., Biochem J. 299, 123-128 (1994); Cohen and Frame, Nat. Rev. Mol. Cell. Biol., 2, 769-76 (2001)]. However, where the insulin response is impaired in a diabetic patient, glycogen synthesis and glucose uptake fail to increase despite the presence of relatively high blood levels of insulin. This leads to abnormally high blood levels of glucose with acute and chronic effects that may ultimately result in cardiovascular disease, renal failure and blindness. In such patients, the normal insulin-induced inhibition of GSK-3 fails to occur. It has also been reported that GSK-3 is overexpressed in patients with type II diabetes [WO 00/38675]. Therapeutic inhibitors of GSK-3 are therefore useful for treating diabetic patients suffering from an impaired response to insulin.
Apoptosis has been implicated in the pathophysiology of ischemic brain damage (Li et al., 1997; Choi, et al., 1996; Charriaut-Marlangue et al., 1998; Grahm and Chen, 2001; Murphy et al., 1999; Nicotera et al., 1999). Recent publications indicate that activation of GSK-3β may be involved in apoptotic mechanisms (Kaytor and Orr, 2002; Culbert et al., 2001). Studies in rat models of ischemic stroke induced by middle cerebral artery occlusion (MCAO) showed increased GSK-3β expression is following ischemia (Wang et al., Brain Res, 859, 381-5, 2000; Sasaki et al., Neurol Res, 23, 588-92,2001). Fibroblast growth factor (FGF) reduced ischemic brain injury after permanent middle cerebral artery occlusion (MCO) in rats (Fisher et al. 1995; Song et al. 2002). Indeed, the neuroprotective effects of FGF demonstrated in ischemia models in rats may be mediated by a PI-3 kinase/AKT-dependent inactivation of GSK-3β (Hashimoto et al., 2002). Thus, inhibition of GSK-3β after a cerebral ischemic event may ameliorate ischemic brain damage.
GSK-3 is also implicated in mycardial infarction. See Jonassen et al., Circ Res, 89:1191, 2001 (The reduction in myocardial infarction by insulin administration at reperfusion is mediated via Akt dependent signaling pathway); Matsui et al., Circulation, 104:330, 2001 (Akt activation preserves cardiac function and prevents cardiomyocyte injury after transient cardiac ischemia in vivo); Miao et al., J Mol Cell Cardiol, 32:2397, 2000 (Intracoronary, adenovirus-mediated Akt gene delivery in heart reduced gross infarct size following ischemia-reperfusion injury in vivo); and Fujio et al., Circulation et al., 101:660, 2000 (Akt signaling inhibits cardiac myocyte apoptosis in vitro and protects against ischemia-reperfusion injury in mouse heart).
GSK-3 activity plays a role in head trauma. See Noshita et al., Neurobiol Dis, 9:294, 2002 (Upregulation of Akt/PI3-kinase pathway may be crucial for cell survival after traumatic brain injury) and Dietrich et al., J Neurotrauma, 13:309, 1996 (Posttraumatic administration of bFGF significantly reduced damaged cortical neurons & total contusion volume in a rat model of traumatic brain injury).
GSK-3 is also known to play a role in psychiatric disorders. See EldarFinkelman, Trends Mol Med, 8:126, 2002; Li et al., Bipolar Disord, 4:137, 2002 (LiCl and Valproic acid, anti-psychotic, mood stabilizing drugs, decrease GSK3 activities and increase beta-catenin) and Lijam et al., Cell, 90:895, 1997 (Dishevelled KO mice showed abnormal social behavior and defective sensorimotor gating. Dishevelled, a cytoplamic protein involved in WNT pathway, inhibits GSK3beta activities).
It has been shown that GSK3 inhibition by lithium and valproic acid induces axonal remodeling and change synaptic connectivity. See Kaytor & Orr, Curr Opin Neurobiol, 12:275, 2002 (Downregulation of GSK3 causes changes in mirotubule-associated proteins: tau, MAPI & 2) and Hall et al., Mol Cell Neurosci, 20:257, 2002 (Lithium and valproic acid induces the formation of growth cone-like structures along the axons).
GSK-3 activity is also associated with Alzheimer's disease. This disease is characterized by the presence of the well-known β-amyloid peptide and the formation of intracellular neurofibrillary tangles. The neurofibrillary tangles contain hyperphosphorylated Tau protein, in which Tau is phosphorylated on abnormal sites. GSK-3 has been shown to phosphorylate these abnormal sites in cell and animal models. Furthermore, inhibition of GSK-3 has been shown to prevent hyperphosphorylation of Tau in cells [Lovestone et al., Curr. Biol., 4, 1077-86 (1994); and Brownlees et al., Neuroreport 8, 3251-55 (1997); Kaytor and Orr, Curr. Opin. Neurobiol., 12, 275-8 (2000)]. In transgenic mice overexpressing GSK3, significant increased Tau hyperphosphorylation and abnormal morphology of neurons were observed [Lucas et al., EMBO J, 20:27-39 (2001)]. Active GSK3 accumulates in cytoplasm of pretangled neurons, which can lead to neurofibrillary tangles in brains of patients with AD [Pei et al., J Neuropathol Exp Neurol, 58, 1010-19 (1999)]. Therefore, inhibition of GSK-3 slows or halts the generation of neurofibrillary tangles and thus treat or reduce the severity of Alzheimer's disease.
Evidence for the role GSK-3 plays in Alzheimer's disease has been shown in vitro. See Aplin et al (1996), J Neurochem 67:699; Sun et al (2002), Neurosci Lett 321:61 (GSK3b phosphorylates cytoplasmic domain of Amyloid Precursor Protein (APP) and GSK3b inhibition reduces Ab40 & Ab42 secretion in APP-transfected cells); Takashima et al (1998), PNAS 95:9637; Kirschenbaum et al (2001), J Biol Chem 276:7366 (GSK3b complexes with and phosphorylates presenilin-1, which is associated with gamma-secretase activity in the synthesis of Aβ from APP); Takashima et al (1998), Neurosci Res 31:317 (Activation of GSK3b by Ab(25-35) enhances phosphorylation of tau in hippocampal neurons. This observation provides a link between Aβ and neurofibrillary tangles composed of hyperphosphorylated tau, another pathological hallmark of AD); Takashima et al (1993), PNAS 90:7789 (Blockade of GSK3b expression or activity prevents Ab-induced neuro-degeneration of cortical and hippocampal primary cultures); Suhara et al (2003), Neurobiol Aging. 24:437 (Intracellular Ab42 is toxic to endothelial cells by interfering with activation of Akt/GSK3b signaling-dependent mechanism); De Ferrari et al (2003) Mol Psychiatry 8:195 (Lithium protects N2A cells & primary hippocampal neurons from Aβ fibrils-induced cytotoxicity, & reduced nuclear translocation/destabilization of b-catenin); and Pigino et al., J Neurosci, 23:4499, 2003 (The mutations in Alzheimer's presenilin 1 may deregulate and increase GSK-3 activity, which in turn, impairs axonal transport in neurons. The consequent reductions in axonal transport in affected neurons can ultimately lead to neurodegeneration).
Evidence for the role GSK-3 plays in Alzheimer's disease has been shown in vivo. See Yamaguchi et al (1996), Acta Neuropathol 92:232; Pei et al (1999), J Neuropath Exp Neurol 58:1010 (GSK3b immunoreactivity is elevated in susceptible regions of AD brains); Hernandez et al (2002), J Neurochem 83:1529 (Transgenic mice with conditional GSK3b overexpression exhibit cognitive deficits similar to those in transgenic APP mouse models of AD); De Ferrari et al (2003) Mol Psychiatry 8:195 (Chronic lithium treatment rescued neurodegeneration and behavioral impairments (Morris water maze) caused by intrahippocampal injection of Aβ fibrils.); McLaurin et al., Nature Med, 8:1263, 2002 (Immunization with Aβ in a transgenic model of AD reduces both AD-like neuropathology and the spatial memory impairments); and Phiel et al (2003) Nature 423:435 (GSK3 regulates amyloid-beta peptide production via direct inhibition of gamma secretase in AD tg mice).
Presenilin-1 and kinesin-1 are also substrates for GSK-3 and relate to another mechanism for the role GSK-3 plays in Alzheimer's disease, as was recently described by Pigino, G., et al., Journal of Neuroscience (23:4499, 2003). It was found that GSK3beta phosphorylates kinsesin-1 light chain, which results in a release of kinesin-1 from membrane-bound organelles, leading to a reduction in fast anterograde axonal transport (Morfini et al., 2002). The authors suggest that the mutations in PSI may deregulate and increase GSK-3 activity, which in turn, impairs axonal transport in neurons. The consequent reductions in axonal transport in affected neurons ultimately leads to neurodegeneration.
GSK-3 is also associated with amyotrophic lateral sclerosis (ALS). See Williamson and Cleveland, 1999 (Axonal transport is retarded in a very early phase of ALS in MSOD1 mice); Morfini et al., 2002 (GSK3 phosphorylates kinesin light chains and inhibit anterograde axonal transport); Warita et al., Apoptosis, 6:345, 2001 (The majority of spinal motor neurons lost the immunoreactivities for both P13-K and Akt in the early and presymptomatic stage that preceded significant loss of the neurons in this SODI tg animal model of ALS); and Sanchez et al., 2001 (The inhibition of PI-3K induces neurite retraction mediated by GSK3 activation).
GSK-3 activity is also linked to spinal cord and peripheral nerve injuries. It has been shown that GSK3 inhibition by lithium and valproic acid can induce axonal remodeling and change synaptic connectivity. See Kaytor & Orr, Curr Opin Neurobiol, 12:275, 2002 (Downregulation of GSK3 causes changes in mirotubule-associated proteins: tau, MAPI & 2) and Hall et al., Mol Cell Neurosci, 20:257, 2002 (Lithium and valproic acid induces the formation of growth cone-like structures along the axons). See also Grothe et al., Brain Res, 885:172, 2000 (FGF2 stimulate Schwann cell proliferation and inhibit myelination during axonal growth); Grothe and Nikkhah, 2001 (FGF-2 is up regulated in the proximal and distal nerve stumps within 5 hours after nerve crush); and Sanchez et al., 2001 (The inhibition of PI-3K induces neurite retraction mediated by GSK3 activation).
Another substrate of GSK-3 is β-catenin, which is degraded after phosphorylation by GSK-3. Reduced levels of β-catenin have been reported in schizophrenic patients and have also been associated with other diseases related to increase in neuronal cell death [Zhong et al., Nature, 395, 698-702 (1998); Takashima et al., PNAS, 90, 7789-93 (1993); Pei et al., J. Neuropathol. Exp, 56, 70-78 (1997); and Smith et al., Bio-org. Med. Chem. 11, 635-639 (2001)]. Furthermore, β-catenin and Tcf-4 play a dual role in vascular remodeling by inhibiting vascular smooth muscle cell apoptosis and promoting proliferation (Wang et al., Circ Res, 90:340, 2002). Accordingly, GSK-3 is associated with angiogenic disorders. See also Liu et al., FASEB J, 16:950, 2002 (Activation of GSK3 reduces hepatocyte growth factor, leading to altered endothelial cell barrier function and diminished vascular integrity) and Kim et al., J Biol Chem, 277:41888, 2002 (GSK3beta activation inhibits angiogenesis in vivo using Matrigel plug assay: the inhibition of GSK3beta signaling enhances capillary formation).
Association between GSK-3 and Huntington's disease has been shown. See Carmichael et al., J Biol. Chem., 277:33791, 2002 (GSK3beta inhibition protect cells from poly-glutamine-induced neuronal and non-neuronal cell death via increases in b-catenin and its associated transcriptional pathway). Overexpression of GSK3 reduced the activation of heat shock transcription factor-i and heat shock protein HSP70 (Bijur et al., J Biol Chem, 275:7583, 2000) that are shown to decrease both poly-(O) aggregates and cell death in in vitro HD model (Wyttenbach eta l., Hum Mol Genet, 11:1137, 2002).
GSK-3 effects the levels of FGF-2 and their receptors are increased during remyelination of brain aggregate cultures remyelinating rat brains. See Copelman et al., 2000, Messersmith, et al., 2000; and Hinks and Franklin, 2000. It was also found that FGF-2 induces process outgrowth by oligodendrocytes implicating involvement of FGF in remyelination (Oh and Yong, 1996; Gogate et al., 1994) and that FGF-2 gene therapy has shown to improve the recovery of experimental allergic encephalomyelitis (EAE) mice (Ruffini, et al., 2001).
GSK-3 has also been associated with hair growth because Wnt/beta-catenin signaling is shown to play a major role in hair follicle morphogenesis and differentiation (Kishimotot et al. Genes Dev, 14:1181, 2000; Millar, J Invest Dermatol, 118:216, 2002). It was found that mice with constitutive overexpression of the inhibitors of Wnt signaling in skin failed to develop hair follicles. Wnt signals are required for the initial development of hair follicles and GSK3 constitutively regulates Wnt pathways by inhibiting beta-catenin. (Andl et al., Dev Cell 2:643, 2002). A transient Wnt signal provides the crucial initial stimulus for the start of a new hair growth cycle, by activating beta-catenin and TCF-regulated gene transcription in epithelial hair follicle precursors (Van Mater et al., Genes Dev, 17:1219, 2003)
Because GSK-3 activity is associated with sperm motility, GSK-3 inhibition is useful as a male contraceptive. It was shown that a decline in sperm GSK3 activity is associated with sperm motility development in bovine and monkey epididymis (Vijayaraghavan et al., Biol Reprod, 54: 709, 1996; Smith et al., J Androl, 20:47, 1999). Furthermore, tyrosine & serine/threonine phosphorylation of GSK3 is high in motile compared to irnmotile sperm in bulls (Vijayaraghavan et al., Biol Reprod, 62:1647, 2000). This effect was also demonstrated with human sperm (Luconi et al., Human Reprod, 16:1931, 2001).
Considering the lack of currently available treatment options for the majority of the conditions associated with GSK-3 protein kinase, there is still a great need for new therapeutic agents that inhibit this protein target.